enantiomers, bias in terrestrial life

All organisms on Earth use amino acids and sugars in only one of the two possible mirror-image
forms (see enantiomers): the amino-acids are all
left-handed (L-) and the sugars are all right-handed (D-). This has prompted
theorists to speculate on how such a bias came about. One possibility is
that the inorganic molecules which provided a substrate on which prebiotic
chemicals assembled in pools on the young Earth were, themselves, of a particular
handedness. Another theory is that the choice was initially made in space,
by the action of circular polarized light,
before even the solar system took shape (see amino
acids, in space).

Research, published in 1998, by Keno Soai and his colleagues at the Science
University of Tokyo,1 supports the view that once an enantiomer
imbalance of even a few percent had arisen it would be amplified by subsequent
chemical reactions. Soai's team examined a mixture of compounds containing
a slight excess of one enantiomer of the amino acid leucine. The components
of the solution reacted to give a substance known as pyrimidyl alkanol,
also with a slight excess of one enantiomer. This molecule, however, then
acted to catalyze its own formation (see autocatalysis),
so that quickly almost all the pyrimidyl alkanol in the solution was of
the same handedness. Although pyrimidyl alkanol is not itself relevant to
prebiological development, its autocatalyzing behavior shows how a minor
enantiomer imbalance could quickly give rise to the bias found in living
systems. The fact that a choice was made has proved vital, because proteins (which are made from amino acids) can fold consistently only if made entirely
of one form or another, but not both.